Workers are being challenged to dress brightly on the third Monday of the month in a bid to combat "blue Monday".

Researchers claim that the third Monday of January is the most depressing day of the year.

Mental Health Research UK (MHRUK) said that thanks to a combination of bad weather, debt, the need for Christmas detox and poor motivation, the nation's collective wellbeing is expected to sink to an all-year low on January 21.

The charity aims to raise awareness of depression and seasonal affective disorder (SAD) with its new campaign, Blooming Monday. It is calling on people to dress in colourful clothing to highlight the plight of those who suffer from the conditions and to raise money for research into treatments.

Dr Laura Davidson, mental health barrister and trustee of MHRUK, said: "It is estimated that one million working hours are lost each year due to SAD. The common work culture - especially in London where lunch breaks are frowned upon - may be contributing to the increasing numbers of those suffering from SAD.

"During the winter I was waiting for a train when another passed by. I counted only four people on the entire train who were not wearing grey or black - and they were still wearing dark colours. We want to encourage people to brighten up for a day - to make the commute to work a kaleidoscope of colour."

People who take part in the event are being encouraged to make a voluntary donation of £2 to MHRUK by texting BLOO22 to 70070.

Dr Davidson, one of the founding members of MHRUK, said the charity was set up in 2008 to fund research into the causes of mental illness.

She added: "When I discovered that no mental health charity existed which focused on trying to change things for future patients by funding innovative research, it was hard to believe. There is a dearth of new research due to restricted funding, and the pharmaceutical companies are pulling away from research into mental illness and the brain because they want quicker results with less expenditure.

"Mental Health Research UK has now funded three research scholarships into mental illness via a competitive process, with each scholarship worth £120,000. No one could suggest that we will unravel the mysteries of the human brain quickly, but we have to start now."